1. Field of the Invention
This invention relates to a sector servo system magnetic disk drive which reads a data signal and a servo signal by the same head. More particularly, this invention relates to a signal processing system having a read channel in which low-frequency cutoff is set relatively high so as to cope with a thermal asperity that is observed in a magnetic disk drive using a magnetoresistance effect type head (MR head) as the head.
2. Description of the Related Art
Recently, a technology for reducing a floating distance of an MR head has been employed to increase the recording density. On the other hand, microscopic protuberances frequently develop on the surface of a disk medium due to changes with time. In such a case, these protuberances collide with the MR head and the heat friction occurring thereby causes fluctuation of the waveform of a reproduction signal (which is referred to as "thermal asperity"). To cope with such a thermal asperity, a low-frequency cutoff frequency has been set to a higher level than in ordinary cases in the past. In a sector servo system magnetic disk, however, both the data signal and the servo signal receive the same low-frequency cutoff. Consequently, the problem occurs in that the waveform of the servo signal using a lower frequency band than the data signal undergoes distortion and the accuracy of positioning the head, which is effected by demodulating this servo signal, drops. Therefore, a technology which prevents such a problem, even when the low-frequency cutoff frequency is set high as a counter-measure to thermal asperity, has been required.
FIG. 1 exemplarily shows the constitution of a signal processing system of a sector servo system magnetic disk drive as a example of the prior art technology.
In the drawing, symbol P1 represents a signal output terminal from which a data signal and a servo signal reproduced by a head (not shown) are outputted. The data signal and the servo signal are obtained by reproducing the data recorded on tracks formed into concentric circles on a disk (not shown) and servo information so recorded as to divide the data areas on the tracks, respectively, and they are outputted to the signal output terminal P1 at mutually different timings. Incidentally, the signal waveform F1 shown in the drawing represents the waveform of the servo signal, and it has a frequency component lower (by about 1/10) than that of the data signal.
Reference numeral 50 denotes an AC coupling portion for imparting low-frequency cutoff characteristics to the reproduction signal outputted to the signal output terminal P1 of the head, and reference numeral 51 denotes an AGC-and-equalization circuit. As shown in the drawing, the AC coupling portion 50 comprises a coupling capacitance C and an input equalization resistance R of the AGC-and-equalization circuit 51. Therefore, the low-frequency cutoff characteristics given to the reproduction signal are determined by the time constant of the capacitance C and the resistance R that constitute this AC coupling portion 50.
Reference numeral 52 denotes a data demodulation circuit, reference numeral 53 denotes a servo demodulation circuit, and symbol P2 represents a branch portion between the data modulation circuit 52 and the servo demodulation circuit 53. Among the reproduction signals which are subjected to a predetermined signal amplification and to waveform equalization, the data signal is demodulated and digitized by the data demodulation circuit 52 and is outputted to a host controller (not shown). On the other hand, the servo signal is demodulated by the servo demodulation circuit 53 (or demodulated and digitized) and is sent to a head positioning mechanism. In this way, the head is positioned to any of the tracks on the disk.
According to the prior art technology described above, a circuit portion (AC coupling portion 50 and AGC-and-equalization circuit 51) having transfer characteristics which determine the low-frequency cutoff characteristics exists in the transfer line ranging from the signal output terminal P1 of the head to the branch portion P2 between the data demodulation circuit 52 and the servo demodulation circuit 53. Therefore, both of the data signal and the servo signal outputted to the signal output terminal P1 of the head receive the same low-frequency cutoff. The low-frequency cutoff characteristics are determined by the time constant (CR) of the AC coupling portion 50 as described above.
It has been customary to set the low-frequency cutoff frequency relatively high as a counter-measure against the thermal asperity as described already. To set this low-frequency cutoff frequency to a high level, the time constant (CR) of the AC coupling portion 50 must be set to a small value.
When the time constant (CR) is set to a small value, however, distortion of the waveform occurs due to a phase change, etc, of the low-frequency cutoff characteristics in the servo signal using a lower frequency band than the data signal (see signal waveform F2 in FIG. 1). In consequence, the problem develops in that head positioning accuracy of this servo signal drops.